I got the fluctuating hash rates with the 8700 as well. On different PC's but with the same hardware. Sometimes 400, 350 or 280 - sometimes even as low as 150. Close and re-open and it can change. Tried with XMR Stak, 2.5 and 2.7. Also tried XMRIG. ll the same.
Auto configs or manually configs?
Use JCE Miner for CPU. Best monero miner for CPU.